1. Field of the Invention
The present invention relates to a speaker system, and more particularly, to a thin-type speaker for the reproduction of deep bass sounds.
2. Description of the Prior Art
It is commonly desired that audio apparatus be compact and of a small depth to minimize space requirements, and yet be capable of reproducing high-fidelity sounds.
However, conventional speaker systems produce deep bass sounds using a relatively large diaphragm and an enclosure of substantial dimensions.
The inventors of the present invention have developed a novel speaker system having a diaphragm installed in a combination enclosure of a fully enclosed cabinet structure and a rear opening cabinet arrangement. Such a speaker system will be described with reference to FIGS. 6(a), 6(b) and 7. FIG. 6(a) illustrates a plan view of the speaker system and FIG. 6(b) illustrates a cross sectional view taken along the line 20-20 of FIG. 6(a). (FIG. 6(a) does not depict a diaphragm for simplicity purposes.) As shown, reference numeral 1 denotes a diaphragm which is mounted by an edge member 2 to an enclosure 3 and which is activated by voice coils 5 installed in the magnetic gap of a magnetic circuit 4. An enclosed cabinet region 6 is provided behind the outer peripheral portion 1a of the diaphragm 1. The minimum resonant frequency f.sub.O of the system is thus determined by the weight M of a vibrating system and the stiffness S.sub.B of the enclosed cabinet region 6 and is expressed as: ##EQU1##
Behind the central area 1b of the diaphragm 1, a rear opening cabinet region is formed by a center plate 7, an inner edge 8, and ducts 9 and 10, the rear opening cabinet region being separated from the enclosed cabinet region 6.
The operation of the speaker system having the foregoing arrangement will now be explained.
FIG. 7 is a frequency characteristic diagram associated with the speaker system activated in an anechoic room. The overall sound pressure equals a sum of a sound pressure in the enclosed region and a sound pressure in the rear opening region. At the enclosed region, a corresponding area of the diaphragm is lessened by an extension of the opening region and thus f.sub.O becomes relatively low and is advantageous for the reproduction of a bass sound. However, the reduced diaphragm area causes a mid-range sound to be reproduced to a lesser extent. At the opening region, a phase-inverted sound emitted from the back of the diaphragm is propagated across the ducts to the rear of the enclosure. The rear sound is diffracted to the front and mixed with the direct sound emitted from the front of the speaker system. When the distance from the rear to the front is small, the phase difference between the front and rear sounds becomes small, particularly in the low frequency range, and thus a low frequency component of the reproduced sound will be diminished. On the other hand, a middle range component of the sound is not affected and will thus be relatively increased in sound pressure. Accordingly, the speaker system having both the enclosed type and opening type cabinet arrangements can produce a better sound, which is flat in a wide range of frequencies and high in acoustic pressure, with the enclosed cabinet arrangement enhancing the bass sound and the opening cabinet arrangement enhancing the mid-range sound.
The drawback of the foregoing speaker system is that when it is placed close to the rear wall of a room, most of the rearward sound emitted from the rear opening is reflected on the rear room wall towards the front of the speaker system. This results in an offsetting of bass components of the front sound with the same of the rear sound and thus the reproduced sound will be diminished in the bass range.